Duration distributions for different softness groups of gamma-ray bursts

TL;DR

This study shows that the classification of gamma-ray bursts based on duration is affected by their softness, and proposes a modified classification scheme considering softness groups for better differentiation.

Contribution

It introduces a new analysis linking softness to duration distributions, suggesting a revised classification method for GRBs based on empirical relations.

Findings

Duration distributions are bimodal within softness groups.

The dip position in the distribution shifts with softness.

An empirical function relates softness parameter to burst duration.

Abstract

Gamma-ray bursts (GRBs) are divided into two classes according to their durations. We investigate if the softness of bursts plays a role in the conventional classification of the objects. We employ the BATSE (Burst and Transient Source Experiment) catalog and analyze the duration distributions of different groups of GRBs associated with distinct softness. Our analysis reveals that the conventional classification of GRBs with the duration of bursts is influenced by the softness of the objects. There exits a bimodality in the duration distribution of GRBs for each group of bursts and the time position of the dip in the bimodality histogram shifts with the softness parameter. Our findings suggest that the conventional classification scheme should be modified by separating the two well-known populations in different softness groups, which would be more reasonable than doing so with a single sample. According to the relation between the dip position and the softness parameter, we get an empirical function that can roughly set apart the short-hard and long-soft bursts: $SP = (0.100 \pm 0.028) T_{90}^{-(0.85 \pm 0.18)}$, where $SP$ is the softness parameter adopted in this paper.